1. Field of Invention.
The present invention is directed to a process for removing hydrogen sulfide (H.sub.2 S) from a high pressure gas stream and recovering liquid elemental sulfur therefrom. More particularly the process of the present invention is particularly suited for the treatment of gas streams which contain in excess of 3 percent of H.sub.2 S and are available at high pressures. Natural gas may contain H.sub.2 S levels from one percent to 90 plus percent and may be recovered at pressures of 20 atmospheres gauge (atg) or more. By the process and apparatus of the present invention such natural gas containing three percent or more H.sub.2 S is treated to remove the H.sub.2 S so that the treated gas is suited for commercial use and, in addition, liquid elemental sulfur is produced.
2. Background of the Invention.
Hydrogen sulfide is often present in gas streams as a contaminant which makes the gas less desirable for domestic, commercial, or industrial purposes. This problem is particularly severe in sour natural gas, which is often produced with H.sub.2 S concentration in excess of 5 percent to as high as 90 percent. Over the years, many desulfurization processes have been developed in attempts to free gas streams of hydrogen sulfide.
The commercial process most often used in removal of hydrogen sulfide from an acid gas or sour gas stream and the production of elemental sulfur is the Claus process. In this process, the gas stream containing the acid gas is first treated with a physical or chemical solvent for hydrogen sulfide. This extraction or washing step produces a clean, treated gas stream and an acid gas stream. The acid gas stream, mainly H.sub.2 S, and a controlled stoichiometric quantity of air are fed into a reaction furnace, where one-third of the H.sub.2 S is burned to SO.sub.2. The H.sub.2 S and SO.sub.2 react thermally in gas phase to form elemental sulfur in the furnace. Further, elemental sulfur is catalytically formed in the reactors which follow the sulfur furnace wherein the sulfur is produced according to the Claus reaction. One such commercial process is disclosed in Hydrocarbon Processing, Apr. 1982, p. 109. An additional step is usually required to clean the tail gas from the Claus reactors. Accordingly, the recovery of the sulfur by the so-called Claus process usually requires process steps: first, extraction of the H.sub.2 S; second, the reaction of H.sub.2 S and the SO.sub.2, first thermally in gas phase and then over a catalyst; and third, a tail gas cleanup reducing the H.sub.2 S passed into the atmosphere, to be within restricted U.S. EPA standards.
Another commercial process for the removal of hydrogen sulfide and the partial removal of organic sulfur compounds from natural and industrial gases is the Stetford process. The sour natural or industrial gas is counter-currently washed with an aqueous solution containing sodium carbonate, sodium vanadate and anthraquinone disulfonic acid (ADA). The hydrogen sulfide dissolves in the aqueous solution and is removed to any desired level. The hydrosulfide form reacts with the five-valent state vanadium and is oxidized to elemental sulfur. The aqueous solution for extracting the sour gases is regenerated by air blowing, and the reduced vanadium is restored to the five-valent state through a mechanism involving oxygen transfer via the anthraquionone disulfonic acid. A specific example of this process is set forth in Hydrocarbon Processing, Apr. 1982, p. 112.
Still another process for the conversion of H.sub.2 S to elemental sulfur is the LO-CAT process. This process utilizes an aqueous solution of iron held in solution by organic chelating agents. The aqueous solution containing the chelated iron serves as both a catalyst in the overall reaction of H.sub.2 S with oxygen and takes part in the reactions by transfer of electrons. A more specific description of the process is set forth in Hydrocarbon Processing, Apr. 1985, pp.70-71.
U.S. Pat. No. 4,487,753 discloses a process for producing liquid elemental sulfur from a CO.sub.2 -rich gaseous stream containing H.sub.2 S. The gas, together with at least a stoichiometric amount of gaseous oxygen in the presence of liquid water, is contacted in a fixed bed comprising a catalyst selected from the group consisting of a transition metal phthalocyanine compound dispersed on a support at a specified pH and temperature. The patent discloses a preferred support as activated carbon.
A process which has been disclosed by Townsend and Reid (U.S. Pat. No. 3,170,766 and 1958 Oil Gas J. 56 [Oct. 13]:120), was proposed as a method for high-pressure natural gas desulfurization and production of elemental sulfur in one operation, thus continuing the conventional process of first absorbing hydrogen sulfide in an gaseous alkaline solution (e.g. ethanolamine), followed by processing the stripped acid gases in a Claus type sulfur plant. In the process sulfur is burned to produce S.sub.O 2 which is carried in a concentrated glycol solution. Solid sulfur is produced The glycol reactor is the equivalent of the catalytic converters of the Claus process.
U.S. Pat. No. 4,579,727, issued on the application of several applicants, including the present inventor, discloses a process for recovering elemental sulfur from a hydrogen sulfide containing gas stream by reacting the hydrogen sulfide in the gas stream with a buffered aqueous solution enriched in thiosulfate ions at an initial pH between about 4.5 and 6.5 for a residence time sufficient to react a portion of the hydrogen sulfide to elemental sulfur. The elemental sulfur is then removed and the solution now lean in thiosulfate ions is regenerated by the oxidation of the remaining hydrogen sulfide in the gas stream to deplete the hydrogen sulfide from the gas stream and to regenerate the liquid solution for recycling to the reduction zone.
In many respects, the method of Pat. No. 4,579,727 has advantages for the removal of H.sub.2 S from gas streams and production of sulfur therefrom. However, due to the relatively low reaction rates the process requires large reaction vessels, especially when the H.sub.2 S conversion is required to reduce H.sub.2 S concentrations to very low levels. The major shortcoming of this process is the formation of appreciable quantities of sulfate ions, which leads to the necessity of cooling a large recycle to recover the sodium sulfate by crystalization under refrigeration. Furthermore, with this process, the sulfur product may be contaminated with H.sub.2 S. Finally, if nitrogen contamination of the gas stream is not allowed, it is necessary to use pure oxygen in the oxidation reaction.
The Bureau of Mines developed a process for desulfurizing industrial stack gases that contained SO.sub.2. In Bulletin 686 by the United States Department of the Interior, Bureau of Mines entitled "The Citrate Process for Flue Gas Desulfurization" by W. I. Nissen et al published by the Superintendent of Documents in 1985, a process is disclosed in which SO.sub.2 is absorbed and H.sub.2 S is generated and reacted with the absorbed SO.sub.2 for the formation of sulfur. The process includes six steps including (1) gas cleaning and cooling, (2) SO.sub.2 absorption, (3) sulfur precipitation and solution regeneration, (4) sulfate removal, (5) sulfur recovery, and (6) H.sub.2 S generation. In the chapter labeled Laboratory Investigations, the following absorbents were screened for SO.sub.2 solubility:
______________________________________ Organic absorbents: Aqueous absorbents ______________________________________ Butyl phthalate Citric acid-sodium hydroxide Dimethyl heptanone Diglycol amine Dimethyl aniline Gluconic acid-sodium hydroxide Dioctylphthalate Glycerine Diphenyl cresyl phosphate Levulinic acid-sodium hydroxide Dow Corning 55 silicone Maleic acid-sodium hydroxide Dow Corning 710 silicone Malic acid-sodium hydroxide Ethylene glycol Monethanolamine Flerol TOF Sodium acetate GE SF 96 silicone Sodium acetate-acetic acid GE 1093 silicone Sodium borate Isodecanol Sodium citrate Kerosene Sodium citrate-diglycol amine Monsanto Therminal 66 Sodium citrate-monoethanol- amine Monsanto Therminal 77 Sodium citrate-sulfaline O-toluidine Sodium citrate-triethylene glycol Stauffer 3664A polyester Sodium hydroxide Tetraethylene glycol Sodium sulfite Tributoxy ethyl phosphate Sodium tartrate Tributyl phosphate (TBP) Sodium tetrathionate Tricresyl phosphate Sodium thiosulfate Triethylene glycol Triethanolamine Triphenyl phosphate Trisodium phosphate Triphenyl phosphite Trisodium phosphate-phosphoric Xylidine acid 2, 6, 8-trimethyl nonanone 10 pct diphenylnaphthylamine in TBP 10 pct triethylene glycol in TBP ______________________________________
Of the absorbents screened the Citrate Process used the citric acid-sodium hydroxide or sodium citrate salt system. The sodium acetate absorbent was tested, but rejected on the ground that the high vapor pressure of acetic acid (248.degree. F. boiling point) contributed to excessive reagent losses. The Citrate Process is carried out at low pressures and low temperatures.